1. Field of the Invention
The present invention relates to a phase shifter provided in a high-frequency circuit, and particularly to a phase shifter that shifts a phase in accordance with a frequency band. The present invention also relates to an impedance matching circuit, and particularly to an impedance matching circuit including a phase shifter. The present invention also relates to a multi/demultiplexer, and particularly to a multi/demultiplexer including a phase shifter. The present invention also relates to a communication terminal apparatus including the phase shifter, the impedance matching circuit, or the multi/demultiplexer.
2. Description of the Related Art
In general, in a high-frequency circuit, a phase shifter may be used for impedance matching. In the related art, the phase shifter may be of a high-pass-filter type or a low-pass-filter type, and a circuit constant is determined to obtain a desired phase-shift amount at a desired frequency. For example, a phase shifter including a high-pass-filter-type circuit and a low-pass-filter-type circuit is disclosed in Japanese Unexamined Patent Application Publication No. 2013-98744.
In communication terminal apparatuses and the like typified by a mobile phone terminal, for example, impedance matching is often necessary in a plurality of frequency bands. Considering the case where, as illustrated in FIG. 34, a phase shifter 73 is provided between an impedance matching circuit 72 and a second high-frequency circuit 74 and where impedance matching is performed between a first high-frequency circuit 71 and the second high-frequency circuit 74 by using the phase shifter 73 and the impedance matching circuit 72, for example, the phase shifter is required to have a phase-shift characteristic in accordance with a frequency band in order to perform impedance matching in a plurality of frequency bands.
For example, if impedance matching is performed in both the low band and the high band, it may be necessary to largely shift the phase of one of the bands while the phase of the other band is almost not shifted. There are two phase-shift operations as follows, for example.
(1) The phase of a high-band signal is shifted while the phase of a low-band signal is not shifted.
For example, the pass phase of the low-band signal is about 0° (or 180°), and the pass phase of the high-band signal is about 90°.
(2) The phase of a low-band signal is shifted while the phase of a high-band signal is not shifted.
For example, the pass phase of the low-band signal is about 90°, and the pass phase of the high-band signal is about 0° or 180°.
Note that in FIG. 34, since the reflected wave from the second high-frequency circuit 74 makes a round trip in the phase shifter 73 when seen from the impedance matching circuit 72, the phase-shift amount of the reflected signal in the phase shifter 73 doubles. That is, a phase-shift amount of 90° is necessary in order to obtain a reflected phase of 180°, and a phase-shift amount of 0° or 180° is necessary in order to obtain a reflected phase of 0°.
However, as follows, the related art has not provided a phase shifter that enables phase-shift operations for each frequency band described in the above (1) and (2).
For example, FIG. 31 illustrates an example of a phase-frequency characteristic of a high-pass-filter-type phase shifter illustrated in FIG. 30A. In this example, the phase-shift amount in a low band (700 MHz band) can be 90°, but the phase-shift amount in a high band (2 GHz band) is 30°, not 0°. In addition, FIG. 32 illustrates an example of a phase-frequency characteristic of a low-pass-filter-type phase shifter illustrated in FIG. 30B. In this example, the phase-shift amount in a low band (700 MHz band) can be −90°, but the phase-shift amount in a high band (2 GHz band) is about 100°, not 180°. Furthermore, in both the low band and the high band, the change in the phase-shift amount in the frequency band is large.
FIG. 33 illustrates an example of an insertion-loss characteristic with respect to the phase of the high-pass-filter-type phase shifter illustrated in FIG. 30A. Since the phase-shift amount is 180° around the cut-off frequency, the insertion loss is increased if the phase-shift amount is increased. In addition, in the low-pass-filter-type phase shifter illustrated in FIG. 30B, if a phase-shift amount of about 180° is obtained in a low band, the cut-off frequency is decreased, and the insertion loss in a high band is excessively increased.
As described above, in the filter-type phase shifters of the related art, it has not been possible to perform phase-shift operations for each frequency band described in the above (1) and (2).
On the other hand, in a diplexer or multiplexer provided with filters having different frequency characteristics between a common port and a plurality of individual ports, each filter typically cannot obtain an independent characteristic.
For example, FIG. 36 is a circuit diagram of a diplexer including a high-pass filter HPF and a low-pass filter LPF. In this example, an antenna ANT is connected to a common port of the high-pass filter HPF and the low-pass filter LPF. An individual port of the high-pass filter HPF is connected to a high-band circuit, and an individual port of the low-pass filter LPF is connected to a low-band circuit. The high-pass filter HPF is formed of inductors L11 and L12 connected in shunt to the ground and a series-connected capacitor C11, and the low-pass filter LPF is formed of series-connected inductors L21 and L22 and a shunt-connected capacitor C21.
However, in such a circuit illustrated in FIG. 36, when the impedance of the inductor L11 of the high-pass filter HPF is excessively decreased in a low frequency band, the inductor L11 substantially becomes a short circuit element. Thus, the short circuit element (L11) is connected to the low-pass filter LPF, and isolation between the low-band circuit and the high-band circuit is degraded in the low frequency band.
The above-described problem, which arises in a configuration in which a short circuit substantially occurs in a filter in other frequency bands among certain use frequency bands is connected to a common port, arises similarly in not only a diplexer formed of the combination of the high-pass filter and the low-pass filter but also a multiplexer formed of the combination of a plurality of band-pass filters and the like.